From Halopedia, the Halo wiki
The Beta-5 Division is a cell of the Office of Naval Intelligence's Section Three. Like its parent section, the Beta-5 Division is often responsible for the development and application of top-secret, morally questionable projects. The cell is so secretive that few UNSC personnel, including those within ONI, are aware of its existence.
The Beta-5 Division was charged with operating the SPARTAN-III program prior to the implementation of the Spartan branch, though many SPARTAN-IIIs were placed under the operational command of other services. The Beta-5 Division also operates the Asymmetrical Action Group, which was created late in the Human-Covenant War in order to disrupt, disarm, and dissolve Covenant operations. Though the group was initially composed of unconventional warfare veterans, it eventually recruited Spartans into its ranks; the Beta-5 Division contracted Chalybs Defense Solutions to create a specialized MJOLNIR variant for the AAG's Spartan operators. Beta-5 also operated the Ferret program overseen by Rear Admiral Serin Osman.
The Beta-5 Division possesses extremely sophisticated decryption facilities.
By December 2553, prior to her ascension to the role of CINCONI, Rear Admiral Serin Osman served as the head of the Beta-5 Division. Dr. Henry Glassman was integrated into Beta-5's experimental research group for his work in astrophysics and quantum theory.
During the Battle of Earth, Doctor Veronica Clayton interrogated Frederic-104, questioning his decisions during the defense of Reach, suggesting that his mistakes had damaged the Spartans' ability to adequately defend the planet. The interrogation was interrupted by a furious Fleet Admiral Hood, and over her protests that he had no authority over Beta-5 protocol he ordered Blue Team to deploy to Earth's surface to take part in operations.
In April 2609, the Beta-5 operative who possessed the journal, as well as several artifacts pertaining to Spartan NOBLE Team, died; one of this operative's subordinates sent these contents in January 2610 to his cousin, a fellow Beta-5 operative. He warned his cousin about the dangers that possessing the artifacts could cause, as some individuals within Beta-5 Division would be willing to kill to keep their existence a secret.
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